The present invention relates to a furnace for heat-treating wafer substrates for forming MR (magnetoresistive) heads, GMR (giant magnetoresistive) heads, MRAM (magnetic random access memory), etc. in a magnetic field in their production processes, and a heat treatment method using such a furnace.
A magnetic head generally has a structure in which a plurality of ferromagnetic layers are laminated on a substrate. For instance, the GMR head has a structure comprising non-magnetic insulating layers between ferromagnetic layers. The MRAM head has a structure comprising antiferro magnetic layers, a pinned magnetic layer, a non-magnetic conductive layer and free magnetic layers in this order from the side of a substrate. The pinned magnetic layers are entirely magnetized in one direction.
To magnetize the pinned layer in one direction, it is necessary to heat-treat or anneal a substrate provided with thin magnetic layers in a magnetic field. An oriented magnetic field of 0.5 T (tesla) or more is usually necessary to be applied, and an oriented magnetic field of more than 1.0 T is necessary depending on the materials of the pinned layer. To apply an oriented magnetic field to wafer substrates, a vacuum heat-treating furnace as shown in FIG. 15 has conventionally been used. This vacuum heat-treating furnace comprises a magnetic field-generating coil 113 equipped with a cooling pipe 112, a high-frequency coil 114 disposed inside the coil 113, and a vacuum container 106 for holding a plurality of wafer substrates 110 disposed inside the high-frequency coil 114.
However, the magnetic field-generating means in this heat-treating furnace with a magnetic field is constituted by an electromagnet having a coil, to which as large electric current as 500-800 A should be supplied to generate a magnetic field of 1.0 T or more, unsatisfactory from the aspect of safety. It also needs a facility for using large electric power, taking large electricity cost for generating a magnetic field, and a large amount of cooling water should be used to remove heat generated by large electric current. Because of these requirements, it suffers from high treatment cost. Further, because there is an extremely large leaked magnetic flux in the above structure, a large vacant space should be kept in addition to a facility space for the sake of safely, and the apparatus should be enclosed by a magnetic body such as iron, permalloy, etc. to prevent influence on ambient electronic equipment, taking the danger to human bodies into consideration.
With a superconductive coil, a magnetic field can be generated without using a large amount of electric power. Though the consumption of exciting current can be made smaller when a superconductive coil is used than when the electromagnet is used, liquid nitrogen or helium should always be consumed to keep superconductivity, resulting in high operation cost. Also, in a system using a superconductive coil, the variation of a magnetic field turns superconductivity to normal conductivity locally, resulting in heat generation in the coil, and if this state were left to stand, the superconductivity of the entire apparatus would be destroyed. Though the superconductive coil can generate as strong a magnetic field as several teslas to several tens of teslas, the range of a strong leaked magnetic field expands in proportion to its magnetic field strength like the electromagnet. Accordingly, it suffers from the problem of a leaked magnetic field like the electromagnet.
What can properly change a magnetic field strength without using exciting current is a Halbach-type magnetic circuit constituted by a combination of a plurality of permanent magnet segments having substantially the same magnetic force with different magnetization directions. For instance, see Journal of Applied Physics, Vol. 86, No. 11, Dec. 1, 1999, and Journal of Applied Physics, Vol. 64, No. 10, Nov. 15, 1988, and Japanese Patent Laid-Open No. 6-224027.
FIG. 16 shows one example of a Halbach-type magnetic circuit. The circular, Halbach-type magnetic circuit shown in FIG. 16 is constituted by an inner, ring-shaped, permanent magnet assembly 1 and an outer, ring-shaped, permanent magnet assembly 2, which are rotatable to each other. When the inner, ring-shaped, permanent magnet assembly 1 and the outer, ring-shaped, permanent magnet assembly 2 are at positions shown in FIG. 16(a), the magnetic field direction of the inner, ring-shaped, permanent magnet assembly 1 is the same as that of the outer, ring-shaped, permanent magnet assembly 2. Accordingly, there is a synthesized magnetic field having strength and direction shown by the arrow, which is formed by combining a magnetic field generated from the inner, ring-shaped, permanent magnet assembly 1 and a magnetic field generated from the outer, ring-shaped, permanent magnet assembly 2, in a center hole 20 of the inner, ring-shaped, permanent magnet assembly 1.
On the other hand, in a state as shown in FIG. 16(b) in which the outer, ring-shaped, permanent magnet assembly 2 has been rotated by 180xc2x0 from the position of FIG. 16(a), the magnetic field generated from the magnetic circuit of the inner, ring-shaped, permanent magnet assembly 1 is offsetting the magnetic field generated from the magnetic circuit of the outer, ring-shaped, permanent magnet assembly 2 because of opposite magnetization directions. Accordingly, there is substantially no magnetic field in the center hole 20. Thus, the strength of the magnetic field can be adjusted from substantially zero to maximum by the rotation angle of both rings.
When the articles to be heat-treated are wafer substrates having magnetic resistance layers, as large a magnetic field as 1.0 T or more is usually needed to stably improve the magnetic resistance effect, and the magnetic field should be uniform and in parallel with the magnetization direction of the thin magnetic layers. However, a conventional heat-treating furnace comprising an electromagnet fails to generate a uniform magnetic field in parallel with the thin magnetic layers.
Accordingly, an object of the present invention is to provide a small, high-safety, high-accuracy, heat-treating furnace with a uniform parallel magnetic field and reduced magnetic field leakage.
Another object of the present invention is to provide a method for heat-treating articles in a magnetic field using such a heat-treating furnace.
The inventors have found that when a plurality of articles are heat-treated or annealed at a time in a magnetic field, permanent magnets can be used for a magnetic field-generating means by providing a cooling means around a means for heating the articles, and that by using a double-ring-type, Halbach-type magnetic circuit as the magnetic field-generating means, a high-accuracy, uniform parallel magnetic field can be applied to the articles in a radial direction during heat treatment. The present invention has been completed based on these findings.
The first heat-treating furnace with a magnetic field of the present invention comprises (a) a magnetic field-generating means constituted by one ring-shaped, permanent magnet assembly comprising a plurality of permanent magnet segments combined with their magnetization directions oriented such that a magnetic flux flows in a diameter direction; and (b) a heat treatment means disposed in a center hole of the ring-shaped, permanent magnet assembly and comprising a cooling means, a heating means, and a heat-treating container for containing heat-treating holder for holding a plurality of articles to be heat-treated in this order from outside.
The ring-shaped, permanent magnet assembly preferably has an inner diameter of 120 mm or more, an outer diameter of 300 mm or more, and an axial length of 100 mm or more. The ring-shaped, permanent magnet assembly has a shorter axial length as it goes outside in a radius direction.
Each permanent magnet segment constituting the ring-shaped, permanent magnet assembly has a residual magnetic flux density of 1.1 T or more and coercivity of 1114 kA/m (14 kOe) or more.
The axial length H and the outer diameter D1 of the ring-shaped, permanent magnet assembly meet the requirement of 2xe2x89xa6D1/Hxe2x89xa610.
The second heat-treating furnace with a magnetic field of the present invention comprises (a) a magnetic field-generating means constituted by an outer, ring-shaped, permanent magnet assembly comprising a plurality of permanent magnet segments combined with their magnetization directions oriented such that a magnetic flux flows in a diameter direction, and an inner, ring-shaped, permanent magnet assembly disposed inside the outer, ring-shaped, permanent magnet assembly and comprising a plurality of permanent magnet segments combined with their magnetization directions oriented such that a magnetic flux flows in a diameter direction; and (b) a heat treatment means disposed in a center hole of the inner, ring-shaped, permanent magnet assembly and comprising a cooling means, a heating means, and a heat-treating container containing heat-treating holder for holding a plurality of articles to be heat-treated in this order from outside.
In the first and second heat-treating furnace with a magnetic field, the inside of the heat-treating furnace is preferably in vacuum, though the degree of vacuum is not restrictive. The heat-treating furnace may contain a small amount of an inert gas.
In the first and second heat-treating furnace with a magnetic field, the cooling means preferably comprises a cooling pipe through which a cooling liquid flows, and a heat sink plate disposed outside the cooling pipe and inside the inner, ring-shaped, permanent magnet assembly.
In the first and second heat-treating furnace with a magnetic field, an axial center of a magnetic field of the magnetic field-generating means is substantially identical with an axial center of an assembly of a plurality of articles to be heat-treated, which are held in the heat-treating container.
The inner, ring-shaped, permanent magnet assembly and the outer, ring-shaped, permanent magnet assembly are preferably rotatable relative to each other, and the articles to be heat-treated in the center hole and the inner, ring-shaped, permanent magnet assembly are preferably not changeable in their relative directions. Because the inner, ring-shaped, permanent magnet assembly and the outer, ring-shaped, permanent magnet assembly are rotatable relative to each other, a magnetic field in the center hole is changeable in a range of 0-2 T.
It is preferable that the inner, ring-shaped, permanent magnet assembly has an inner diameter of 120 mm or more, that the outer, ring-shaped, permanent magnet assembly has an outer diameter of 300 mm or more, and that the inner, ring-shaped, permanent magnet assembly or the outer, ring-shaped, permanent magnet assembly has an axial length of 100 mm or more.
In another preferred embodiment of the present invention, the inner, ring-shaped, permanent magnet assembly and the outer, ring-shaped, permanent magnet assembly have different axial lengths.
In a further preferred embodiment of the present invention, the inner, ring-shaped, permanent magnet assembly and/or the outer, ring-shaped, permanent magnet assembly have shorter axial length as it goes outside in a radius direction.
Each permanent magnet segment constituting the outer, ring-shaped, permanent magnet assembly and the inner, ring-shaped, permanent magnet assembly preferably has a residual magnetic flux density of 1.1 T or more and coercivity of 1114 kA/m (14 kOe) or more.
The axial length H of the inner, ring-shaped, permanent magnet assembly and the outer diameter D2 of the outer, ring-shaped, permanent magnet assembly preferably meet the requirement of 2xe2x89xa6D2/Hxe2x89xa610.
The method for heat-treating a plurality of articles at a time in a magnetic field, using the above heat-treating furnace with a magnetic field, comprises the steps of (1) introducing a heat-treating holder, on which a plurality of the articles to be heat-treated are placed, into the heat-treating container at a relative rotation position of the inner, ring-shaped, permanent magnet assembly and the outer, ring-shaped, permanent magnet assembly at which a radial magnetic field is substantially zero in the center hole; (2) heat-treating the articles in the heat-treating container by the heating means while cooling the magnetic field-generating means by the cooling means, in a state where a predetermined magnetic field is caused to exist in the center hole by rotating the outer, ring-shaped, permanent magnet assembly relative to the inner, ring-shaped, permanent magnet assembly; and (3) after the completion of heat-treating the articles, taking a plurality of heat-treated articles out of the heat-treating container at a relative rotation position of the inner, ring-shaped, permanent magnet assembly and the outer, ring-shaped, permanent magnet assembly at which a radial magnetic field is substantially zero in the center hole.
The article to be heat-treated is preferably a wafer substrate having thin magnetic layer on the surface.
An assembly of the articles to be heat-treated is held in the heat-treating container, preferably at a position at which an axial center of an assembly of a plurality of articles being heat-treated is substantially identical with an axial center of a magnetic field of the magnetic field-generating means.
The heat treatment is carried out preferably when the heat-treating container is substantially in a vacuum state.